Our research is concerned with the early steps in the metabolism of inorganic sulfur and nitrogen compounds. Major emphasis is on ATP sulfurylase and APS kinase from the filamentous fungus Penicillium chrysogenum. The fungal ATP sulfurylase has been purified to homogeneity and partially characterized kinetically, physically, and chemically. Our current efforts are aimed at (a) elucidating the mechanism of the reaction, (b) identifying the amino acid residues responsible for the functional integrity of the enzyme, and (c) exploring the potential interaction of ATP sulfurylase with APS kinase. Parallel studies are in progress on the ATP sulfurylase from Penicillium duponti (a thermophile). Our work with the P. duponti enzyme may shed some light on the factors responsible for the remarkable heat stability of this enzyme. APS kinase, the second sulfate activating enzyme has been purified to homogeneity from P. chrysogenum and is being characterized. Studies on the sulfate reduction pathway of fungi will be initiated. Other proposed projects include studies on nitrate reductase and nitrite reductase from P. chrysogenum, with particular emphasis on the kinetic mechanisms of these enzymes. Some of the methods that will be employed include (a) analysis of steady-state and reaction progress kinetics, (b) equilibrium ligand binding with native and chemically modified enzymes, and (c) chemical modification of specific amino acid residues.